That's not "degrees of uselessness" that is "completely totally useless and you might as well not play." You LITERALLY said repeatedly you might as well not play. So do not pretend you're making some "degrees of uselessness" now. You took an extreme position, so you better own it.
Fine. I shouldn't have said that the barbarian might use a bow, given how little of an improvement it is over using a javelin. They're both useless, and everyone would have more fun if the barbarian sits out that fight. (Unless the barbarian has already invested heavily in Dex, in which case the bow would be a significant improvement, but we can't make that assumption anymore than we can assume multiclassing into warlock.)
It's not a house rule that you can use two javelins a round for 3 out of four rounds. You just didn't want to reply to that post.
It would be a house rule to suggest that you can draw two in a round, and still attack. If you spend an action to prepare before combat, then that's an exception to the rule, just as if you'd spent that action before combat to drink a Fly potion; there were extenuating circumstances which made you less terrible than you usually are. Once you start applying pre-conditions, though, the argument loses what applicability it still had. We're already limiting the discussion to important boss fights across unpassable terrain; if you limit the argument to important boss fights across unpassable terrain where you know the geography before the battle starts, then that's getting into a remote corner case.
In my view, you appear to want them to be as good at range as they are at melee and if they are not at least very close to the same they'd be in melee then you might as well go play Mario Kart because there is nothing to interest you about the game anymore (which would get you fired as a player in my game).
There's a limit of efficacy, beyond which the player is probably not having fun anymore. I can't tell you where you should draw that line, but I draw that line at one attack per round, with Disadvantage. (The attack also deals less damage than anyone else is dealing with a single attack, but that's irrelevant at this point.) One attack per round, with Disadvantage, is the equivalent of "probably nothing happens on your turn"; and given how long it can take for your turn to come up, that's not worth looking forward to. I'm not having fun at that point.
If the disparity was less exponential, then that could solve the problem. Reduce the rate of fire,
or the accuracy,
or the damage. Or reduce each one by a small amount, such that the net impact is that you're half as effective. When you cut the rate of fire by half, and the accuracy by half, and the damage by (almost) half, the net impact is not worth the effort of rolling dice.
I don't know where this seemingly entitled attitude concerning you being "awesome always!" in combat is coming from, but most games in my experience have an ebb and flow concering spotlight and sometimes you're "Awesome!" barbarian will not be as effective as the ranged player, and other times they will be more effective than the ranged player, and you're expected to participate and not pout and act bored if you're not doing as much damage as you're used to on rare occasion.
Ebbing and flowing, and sharing the spotlight, is perfectly reasonable over the course of a campaign. The final boss fight is not the time for that. The final boss fight should be the most exciting part of the game for everyone involved. An hour is too long for the spotlight to stay on any one character, and that's twice as true if it's the last spotlight of the campaign.